1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray detector for deriving an optical image from an X-ray image, and an image pick-up apparatus for deriving an image signal from the optical image, which image pick-up apparatus includes an image sensor provided with plurality of sensor elements.
2. Description of Related Art
An X-ray examination apparatus of this kind is known from European patent application EP 0 655 860.
The known X-ray examination apparatus includes an image pick-up apparatus with an adjustable spatial resolution. The image pick-up apparatus of such an X-ray examination apparatus is provided with a semiconductor CCD image sensor in which sensor elements are defined by way of gate electrodes. Incident light is absorbed in the semiconductor material of the individual sensor elements so that electric charges, electrons or holes, are released in the sensor elements. Said electric charges are collected in the sensor elements after which they are transported to a read-out register and the image signal is derived from the electric charges in the read-out register by means of a read-out amplifier.
The CCD image sensor in the known X-ray examination apparatus is notably provided with groups of collecting gate electrodes which are isolated from one another by isolating gate electrodes. The collecting gate electrodes receive an electric voltage whose polarity opposes that of the charge of electric charges collected in the relevant sensor elements. The isolating gate electrodes receive an electric voltage having the same polarity as the electric charges being collected. The spatial resolution of the known image pick-up apparatus is determined by adjusting the gate electrodes to the collecting state or the isolating state. The spatial resolution of the known image pick-up apparatus is thus adjustable. However, when an image containing a large number of pixels, for example 1000.times.1000 pixels, is to be picked up by means of an equally large number of sensor elements, an intricate operation is required to change the spatial resolution of the known image pick-up apparatus, because a very large number of gate voltages must be adapted. It is notably difficult to pick up an image of large dimensions with a high spatial resolution by means of the known X-ray examination apparatus.